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Question

Thomas asks: During open heart surgery, patients are often connected to the heart-lung machine. I know that patients are regularly cooled down this way for reduced metabolism. What is the longest time that one could survive without running blood circulation, and at what temperature in degrees centigrade would metabolism be maximally reduced without the risk of damaging human cells?

Answer

James - During heart surgery it’s important for the heart surgeon that the heart stops beating. A beating heart is very hard to work on in terms of putting in sutures etc. So what the surgeon does is stops the heart beating and this is done using an infusion of potassium and also the patient is cooled down about 4 or 5 degrees lower than normal body temperature. And, finally, they’re connected to what’s called a heart/lung bypass machine which simply, as the name suggests, bypasses the heart circulation. So the patient’s own blood is taken out from the right side of the heart (the atria). It is then oxygenated artificially in the heart/lung machine and then put back into the patient. So the patient’s heart is not beating and this can be sometimes for many hours without any detrimental effects. There’s a second part to the question which is how long could somebody survive without any circulation? We know that the brain is the most sensitive organ to not having adequate oxygen and blood supply, and most people think that two to three minutes without any circulation is enough to cause very severe, irreparable brain damage.

Chris - Now what’s interesting is that you’ve told us all these figures about cells needing oxygen and sugar and things but the egg that we were talking about earlier Maude, that doesn’t have any circulation. So how does it get its oxygen and sugar and why doesn’t the egg succumb to the same problems because chickens are warm blooded and the eggs are incubated under the hens body to keep it warm, aren’t they? So how do they protect themselves?

Maude - What is amazing with an egg is they are self-sufficient, at least for a certain period of time. In the human it’s only self-sufficient for the first 10 days before implantation. Then the egg will implant in the uterus for humans and will develop blood exchange between the mother and the baby.